Subsurface exploration can be desirable for a large number of reasons and can serve for a wide variety of purposes. For example, one may wish to detect the presence, concentration, change of concentration or the like of a substance which is naturally present at a particular below-ground region, or a substance which may appear at such a region because of leeching or migration from another location, as in the case of monitoring waste disposal sites, or one may wish to determine the concentration of a marker substance introduced into the region as an indication of migration of materials through the region. For this purpose, it is customary to provide a well or bore hole and a monitoring unit in the cell or bore holes.
One process for subsurface exploration in which at one location in the ground at least one marker substance is introduced, has been described by M. Schottler in: "Entwicklung eines Einbohrlock-Verfahrens zur Messung horizontaler Grundwasserstromungen" lines 19, 20 of page 1, (Development of a Single Borehole Process for Measurement of Horizontal Ground Water Flows).
In this article, a borehole probe is described for carrying out the process. The borehole probe operates with a measurement cell which contains two light sources, a lens system and a special video camera. A marker substance (tracer) of a material capable of fluorescence is liberated centrally over an observation region on which the system is focussed with a focal length of several millimeters. This region lies axially centrally of the measurement cross section and is freely traversable by the flow to be measured. Light emitted from the light sources are re-emitted light of shifted wavelength from the marker are registered by the video camera as bright image points. A contrasting image point is found on an image plane captured by video techniques.
This borehole probe and the process by which it is used and is effective can be provided in standard 10 cm (4 inch) wide boreholes.
Another borehole probe from GSF Munich, Germany, can detect the movement of radio-active marker substances. For this purpose, however, it is necessary to make use of a short-lived isotope which must be produced in a nuclear reactor. This process is relatively expensive. In addition, because the marker substance has a relatively short life, it is sensitive to storage time and it is difficult to keep available quantities of the marker in stock.
The University of California has developed a bore hole probe in which a sample is pumped from the subterranean region of an above-ground location. This system does not permit in situ measurement.
The Technical University of Freiburg, Germany has also developed a bore hole probe. This system operates with heat pulses and has been found to be suitable only for the determination of high ground water speeds.